This invention relates to electric high-interrupting capacity, current-limiting fuses complying with Underwriters Laboratories Inc. Standard for Class L fuses, generally referred-to by the abbreviation UL 198.2.
Such fuses when carrying 150 percent of their current rating shall clear within 240 minutes.
Class L fuses come in predetermined current ratings and cartridge sizes, more fully set forth in UL 198.2, Table 22.1. The current-carrying capacity of Class L fuses must be high.
The interrupting rating of Class L fuses must be high as spelled out more particularly in UL 198.2, paragraphs 12.29 and 12.33A.
For further requirements of fuses complying with Underwriters Laboratories Inc. Standard for Class L fuses reference may be had to that Standard.
Heretofore all the fusible elements of a Class L fuse were made of silver, or a portion of the fusible elements were made of silver, and the remaining portions were made of copper. The terms silver and copper are used in this context to include silver and copper having impurities, or alloys of silver and copper which behave substantially in the same fashion as commercial pure silver and electrolytic copper.
It is the principal object of the present invention to provide Class L fuses all of the fusible elements of which are of copper.
The resistivity of silver at 20.degree. C. is 1.64.times.10.sup.-6 ohm/cm, and the resistivity of copper at 20.degree. C. is 1.72.times.10.sup.-6 ohm/cm. The fact that the resistivity of copper is higher than that of silver calls for an increase of the total cross-sectional area of the fusible elements of copper in regard to the total cross-sectional area of fusible elements of silver, all other conditions remaining unchanged. In other words, a Class L fuse of a given current rating, all the fusible elements of which are of copper, involves a larger mass of metal than a Class L fuse having the same current rating, but all the fusible elements of which are of silver. The relative increase in mass of fusible element metal raises the problem of de-ionizing and cooling the increased body of metal vapors resulting from blowing of the fuse.
Another factor that must be considered when switching from fusible elements of silver to fusible elements of copper is the difference in ionization potential of silver vapor and copper vapor at given temperatures.
Still another factor that must be considered is the temperature coefficient of resistance of silver and copper, respectively.
The resistance and temperature of a fusible element of copper increase at the same rate as the resistance and temperature of a fusible element of silver when both are subjected to the same conditions, in particular the same current. Hence this aspect of the changeover from silver to copper does not involve any problem.
In fuses having an overlay of a low fusing point metal, such as tin, on a high fusing point base metal, such as silver, or copper, the interdiffusion of the two metals, generally referred to as M-effect, depends upon their temperature. The interdiffusion will occur at a relatively slow rate, at relatively low temperatures, and occur at a relatively high rate, at relatively high temperatures. It is a fact for which several reasons may be assigned that it takes much more time to sever by M-effect fusible elements of copper than fusible elements of silver, all other conditions remaining unchanged.
It is, therefore, a prime object of this invention among the several objects thereof to provide fusible elements of copper suitable for Underwriters Laboratories Inc. Standard Class L fuses, i.e. wherein the M-effect severing process is relatively rapid, or closer to that occurring in Class L fuses having fusible elements of silver. To be more specific, the fusible elements of copper according to this invention must clear within 240 minutes when the fuse is carrying a current 150 percent of its rating, as required by the standard under consideration. Many prior art fusible elements of copper do not clear at all at such small overloads.
Silver has a total i.sup.2 .multidot.t value of 8.00.times.10.sup.8 (amp/cm.sup.2).sup.2 sec., i.e. this is the sum of the i.sup.2 .multidot.t value to bring silver from 20 deg. C. up to the melting point, the i.sup.2 .multidot.t value of the melting period required to supply the latent heat of fusion, the i.sup.2 .multidot.t value to bring the melted silver up to its vaporization temperature and the i.sup.2 .multidot.t value required to supply the latent heat of vaporization. Copper has a total i.sup.2 .multidot.t value of 11.72.times.10.sup.8 (amp/cm.sup.2).sup.2 /sec. Therefore the changeover from fusible elements of silver to fusible elements of copper requires a decrease of the points of reduced cross-sectional area of the latter, all other conditions remaining unchanged.